Self-expanding stent grafts are generally flexible and resistant to kinking. The shape memory properties of self-expanding stent grafts also help them to avoid undergoing permanent deformations. For bridging stents, self-expanding stent structures can be problematic, because the delivery catheter typically needs to be pulled back over the stent to release the stent. The action of pulling back the delivery catheter can result in the stent being dislodged from the target vessel. Also, the amount of chronic outward radial force required for fixation found most often in a balloon expandable stent graft can cause stenosis formation because of a sharp compliance transition.
Active fixation relates to suprarenal portions of abdominal aneurysm stent grafts, but active fixation is achieved by a mechanism tied to the central core of the delivery system. Traditionally, active fixation includes traumatic hooks that are released by actuating a button on the delivery system. This mechanism may be quite bulky and so it can only be used in main body aortic components. The delivery catheter required in such an example is 20 French, which is much larger than the 7 French catheters used with bridging stents.
Passive fixation refers to a securement technique between a stent graft and arterial structure such that fixation through puncture of the arterial structure is avoided. Instead, passive fixation is often achieved through chronic outward radial force of the stent itself Chronic outward radial force is the outward radial force created when an oversized stent or stent graft is placed within an arterial structure. Chronic outward radial force may be problematic for several reasons. For example, in an aortic application, chronic outward radial force may potentially cause aneurysmal growth. In addition, chronic outward radial force applied in a smaller branched vessel may result in extensive stretch at the transition from covered to the uncovered vessel. This stretch can cause damage that leads to vessel narrowing or neointimal hyperplasia. This phenomena can also be referred to as “edge stenosis.”